DC motor drives are utilized in industry to serve a wide variety of purposes and commonly employ an AC to DC phase-controlled thyristor converter for controlling the motor. The converter, through the controlled switching of thyristors, changes the AC supply voltage to a controllable DC output voltage which is applied to the armature windings of the motor. By use of thyristors, commutation (that is the transfer of current from one thyristor to another and hence from one phase of the supply to the next phase) is achieved naturally in that the polyphase AC supply waveforms are responsible for turning off a conducting thyristor in much the same way as a similar circuit of diodes would behave and no special or additional circuitry is required for the commutation process; such commutation is known in the art as natural or line commutation. Thyristor AC/DC converters are therefore relatively simple in principle, though complex circuitry is commonly employed for controlling thyristor ignition, and are widely utilised in many industrial fields.
FIG. 1 of the accompanying drawings is a schematic illustration of a conventional three-phase, phase-controlled, naturally-commutated thyristor bridge, shown typically with a DC motor load. As is well known, control circuitry (which has been omitted from FIG. 1) is employed for providing triggering signals to the thyristors in the bridge at defined time instances in the waveforms of the three phases of the AC supply to determine the signed magnitude of the DC output provided to the motor M. Such control circuitry is commonly sophisticated and might, for example, employ algorithms running in microprocessors, phase locked loops, etc., as is disclosed, for example, in the book "Thyristor DC Drives" by Paresh C. San (published by John Wiley & Sons in 1981).
The circuit of FIG. 1 is known as a full converter in that the motor terminal voltage can be reversed, in dependence upon the triggering of the thyristors, so that the thyristor converter operates in a so-called inversion mode in which power can be transferred from the motor back through the thyristor bridge and into the AC supply. By virtue of this facility, regenerative braking can be accomplished in a controlled manner by means of the illustrated circuit, with the kinetic energy of a drive system coupled to the DC motor being converted into electrical energy by the motor and returned through the thyristor bridge to the AC supply. The circuit of FIG. 1 thus is useful as a controller for a DC motor which enables the motor speed and torque to be precisely determined in forward and in reverse operation of the motor, enabling the motor to be effectively and controllably braked by regeneration.
The bridge arrangement shown in FIG. 1 provides for control only in two quadrants in the armature current/back emf domain. FIG. 2 of the accompanying drawings shows this domain, and it will be readily appreciated by those skilled in the art that control is possible only in the first two quadrants with the circuit of FIG. 1. To achieve full four quadrant control, dual converters in which a similar but oppositely connected thyristor bridge is additionally connected across the motor terminals are used and enable the motor current as well as its terminal voltage to be reversed. Dual converters are widely used in industry to control reversible drives in sheet metal rolling mills, in papermaking machinery, in cablemaking and wire drawing machinery, etc.
Current understanding of the control range of phase controlled, three-phase, six-pulse, thyristor converters has set certain limits beyond which loss of current control may be expected. In practice, these restrictions lead to motor derating through compromising the regenerative voltage span, as will be explained hereinafter. It is without question that the three-phase, six-pulse, fully-controlled, naturally-commutated, phase-controlled thyristor bridge used in conjunction with separately excited direct current machines (in two quadrant and four quadrant control) has provided the mainstay of variable speed (and torque) control to industry. To achieve good control characteristics of torque (motor armature current) and speed, a complete understanding of the highly nonlinear sampled data performance of the thyristor stack is required, and there is confusion in the minds of many users relating to the specification of motor armature voltage, in that it is different for non-regenerative and regenerative applications. It is thus customary to specify armature voltages (at full speed) differently for non-regenerative and regenerative drive applications. Indeed, in Europe, this is actually defined by DIN standards (DIN 40030). Also of great significance is the variable defined as E, namely, motor armature back emf. The variable E is directly proportional to speed, and is sign dependent upon direction of rotation (assuming constant field flux). The motor manufacturer will commonly quote armature terminal voltage on the machine name plate, but the converter designer is also particularly interested in the back emf E, which is of great importance.
Conventionally, the control range of a naturally-commutated, thyristor, phase controlled polyphase bridge as shown in FIG. 1 has been considered to be limited to .pi. radians electrical, and it has been conventional to design the thyristor switching control circuitry to ensure operation within this limitation. FIG. 3 of the accompanying drawings shows the sinusoidal envelope of the per phase line-to-line potential of the polyphase AC supply, and marked in FIG. 3 are the lower phase limit or rectification limit for control of the relevant thyristor of the bridge and the upper phase limit or inversion limit spaced from the rectification limit by .pi. radians. The rectification limit represents the condition which would be achieved if the thyristors were simply replaced by diodes, and corresponds to the cross-over point in the waveform of one phase of the AC supply with the waveform of the next adjacent phase; it can be defined as the phase value beyond which further advance would produce no increase in output power into a resistive load and, for a three phase supply, would produce an average output voltage V.sub.AVE equal to V.sub.LL .sqroot.2.times..sup.3 /.pi. where V.sub.LL is the rms line-to-line voltage. The inversion limit has been defined as being .pi. radians electrical retarded from the rectification limit, since beyond this point commutation will not successfully transfer conduction from the present phase to the next, which may result in a catastrophic let-through to the load and loss of control.
Consequently, it has not been the practice to operate fully-controlled, three-phase, naturally-commutated, thyristor DC motor drives in an inversion mode at thyristor firing phases beyond the inversion limit.
When operating in a regenerative braking inversion mode, control of a DC motor by phase control of a thyristor bridge as hereinabove described is possible only so long as the back emf of the motor (which is sign dependent upon the direction of rotation of the motor, assuming constant field flux) is no greater than the maximum voltage available at the motor terminals from the thyristor bridge. If the back emf developed by the motor exceeds the thyristor bridge output, then all possibilities for control will be lost, with consequent risk of catastrophic failure as control of current is lost. The inversion limit restricts the control possibilities which are available, and in consequence of this DC motors controlled by such thyristor bridges have to be correspondingly de-rated.
The conventionally set inversion limit corresponds to the need to transfer conduction between two phases of the AC supply before the commutating thyristor bias becomes such that it is no longer possible to transfer load current to the commutating thyristor in sequence in order to maintain proper control of the load current; in practice, circuit impedance necessitates a forward displacement of this limit to provide a sufficient volt-second margin to ensure reliable commutation under all reactive supply conditions.
A factor of safety is therefore commonly introduced by defining a so-called inversion end stop. The inversion end stop is commonly phase advanced from the inversion limit by as much as 15.degree. or even more. The phase displacement between the end stop and the inversion limit is determined in dependence upon the inductance of the supply and the amount of current to be commutated. The introduction of such an end stop effectively reduces the maximum available control range from 180.degree. (.pi. radians) to for example 165.degree. . It is the intersection of the end stop and the instantaneous voltage that sets the maximum value of motor back emf into which a controlled amount of current can be achieved.
The result of these compromises is an increase in motor costs due to the non-standard requirements of the armature voltage and a further increase in costs as a result of the required increase in current capacity.
In mainland Europe, the three-phase industrial supply is 380 volts rms, and in the UK it is 415 volts rms. According to DIN standards (in mainland Europe), an armature terminal voltage of 460 volts is defined. This value may be achieved by consideration of the fact that the maximum average (DC) output voltage obtainable from a three-phase, fully-controlled bridge at fully rectification from a 380 V.sub.AC supply is: ##EQU1## where throughout phase-to-phase voltages are used and allowing for supply tolerances of 10%, this gives V.sub.DC =460 V under worst case conditions. On UK supplies, V.sub.DC is equal to 500 V under worst case conditions, but it is not uncommon to find UK standards as 490, 480 and 460 volts DC, which causes confusion and non-standardization of DC motors.